Synthetic T4 is identical to that produced in the human thyroid gland.

Levothyroxine has a 7-day half-life, is absorbed relatively slowly, and equilibrates rapidly in its distribution volume, therefore avoiding significant postabsorptive perturbations in free thyroxine levels. Because of the 7-day half-life, it takes approximately 6 weeks before there is complete equilibration of the free thyroxine levels and the biologic effects of levothyroxine. Dosage is adjusted by following free thyroxine levels and thyrotropin levels, which should normalize in approximately 2 months.

The myopathy improves within 2-3 weeks, but months may be required for it to resolve completely.